In recent years, a SAW device is widely used in the field of communications. Because of its excellent features such as high performance, smallness in size, and mass productivity, the SAW device is often used in devices such as mobile phones and LANs. A SAW device that has been used widely uses the Rayleigh wave ((P+SV) wave) which propagates in an axis X direction on an ST cut quartz substrate (a quartz substrate whose plane XZ (plane Y) is rotated by 42.75° in a counterclockwise direction from a crystal axis Z around a crystal axis X as a rotation axis). Although the primary temperature coefficient of the ST cut crystal SAW device is zero, the secondary temperature coefficient is relatively high at about −0.034 (ppm/° C.2), and it is a problem that the frequency variation becomes great when used in a high temperature range.
To solve this problem, SAW devices have been disclosed in Meirion Lewis, “Surface Skimming Bulk Wave, SSBW”, IEEE Ultrasonics Symp. Proc., pp. 744-752 (1977) and Japanese Examined Patent Application Publication No. 62-016050. These SAW devices are, with reference to FIG. 20a, an SH wave type SAW device in which the cut angle θ of a rotary Y cut quartz substrate is rotated by −50° in a counterclockwise direction from the crystal axis Z (the axes of the substrate after the rotation are represented by axes X, Y′, and Z′) and which utilizes an SH wave propagating in a direction perpendicular to the axis X (axis Z′ direction). To express this cut angle in the Eular angles, it can be expressed as (0°, θ+90°, 90°)=(0°, 40°, 90°). FIG. 20b is an SH wave type SAW resonator that includes: an IDT electrode 82 arranged along the axis Z′ on the main surface of a rotary Y cut quartz substrate 81, and grating reflectors 83a, 83b on both sides of the IDT electrode 82. This SH wave type SAW resonator performs as a resonator in a manner that the SH wave type surface wave propagating directly under the surface of the piezoelectric substrate 81 is excited by the IDT electrode 82, and the vibration energy is trapped directly under the electrodes (82, 83a, 83b). Generally, the SH wave type SAW resonator has good frequency-temperature characteristics showing a tertiary curve in a wide temperature range.
However, this SH wave type surface wave is essentially a wave advancing through the substrate (SSBW). Thus, when compared to a SAW device using a wave like the Rayleigh wave that is excited by the ST cut quartz plate and that propagates along the surface of a piezoelectric substrate, there are problems that the reflection efficiency of the surface acoustic wave by the grating reflectors is low and that it is difficult to produce a small-size SH wave type SAW device having a high Q value.
In order to solve these problems, Japanese Examined Patent Application Publication No. 1-034411 (Patent Document 2) discloses, as shown in FIG. 21, a SAW resonator using an SH wave type surface wave that propagates in the axis Z′ direction on the rotary Y cut quartz substrate 81 of which cut angle θ is −50°. This is a so-called multi-paired IDT electrode type SAW resonator aimed to attain a high Q value that contains 800±200 pairs of IDT electrodes 84 and traps the vibration energy of the SH wave type surface wave by only using reflection from electrode fingers of the IDT electrodes 84 without using the grating reflectors.
However, this multi-paired IDT electrode type SAW resonator has less energy trapping effect in comparison to the ST cut quartz crystal SAW resonator (the Rayleigh wave type) and requires an extremely large number of pairs of IDT electrodes, such as 800±200 pairs, in order to obtain a high Q value. Therefore, it is a problem that the size of the substrate becomes larger than that of the ST cut quartz SAW resonator, thereby increasing the device size and making it impossible to respond to recent requirements for miniaturization.
Also, with the SAW resonator disclosed in Patent Document 2, it is stated that the Q value can be raised by setting an electrode film thickness at 2% λ or more, preferably 4% λ or less, where λ is an electrode period (wavelength) of the SH wave type surface wave excited by the IDT electrodes. For example, if frequency is 200 MHz, the Q value becomes saturated when a standardized electrode film thickness H/λ (a value of electrode film thickness H standardized by wavelength λ, where λ is a wavelength of SAW to be propagated: also called simply as electrode film thickness) is at around 4% λ. Thus, the obtained Q value is only about the same as that using the ST cut quartz SAW resonator. A possible reason for this is that, when the standardized electrode film thickness ranges from 2% λ or more to 4% λ or more, the SH wave type surface wave cannot be trapped in the surface of the piezoelectric substrate, and the reflection efficiency becomes insufficient, thereby not raising the Q value.
Along with requests for a higher frequency for the SAW device, good frequency aging characteristics are strongly demanded, and a number of suggestions have been made to improve the aging characteristics as well as power durability. Japanese Unexamined Patent Application Publication No. 5-199062 (Patent Document 3) offers one such suggestion. According to this document, it is necessary to single-crystallize an aluminum electrode formed on a quartz substrate in order to improve the frequency aging characteristics. It is stated that single-crystallization of the aluminum electrode depends on the surface state of the quartz substrate and prevention of contamination of the substrate surface before growing an aluminum film.
To produce a quartz substrate, an ST cut quartz substrate (33 degrees ST cut quartz substrate) of a rotary Y plate is cut from a quartz block, and both surfaces are subjected to lapping polishing until a predetermined thickness, followed by polishing of these surfaces. Etching is conducted finally to remove work-affected layers off the surfaces of the quartz substrate and to release stress caused by polishing. The process of etching the quartz substrate is conducted by immersing the substrate in an etching solution such as a compound liquid containing hydrofluoric acid or ammonium fluoride and by etching the surface of the quartz substrate by about 0.1 μm to 2 μm. As a result, the substrate surface establishes an island-like structure having evenly laid-out, half-spherical minute islands, with the diameters of the half-spherical islands ranging from 10 nm (nanometers) to 10 nm (nanometers) and the heights thereof ranging from about 1 nm to 20 nm. Also, the intervals between the islands are from about 10 nm to several ten nm. Because the state of the surface varies depending on the etching time and concentration of the etching solution, it is stated that the condition controls are necessary.
As a technique for keeping the quartz substrate clean, the time between etching the substrate and attaching the substrate to a film deposition apparatus should be as short as possible, and the substrate is either steam-dried with isopropyl alcohol or immersed in isopropyl alcohol. Thereafter, the substrate is made hydrophobic by a liquid removal method or the like using centrifugal separation. Further, to avoid contamination of the substrate surface by gas emitted from a wall surface inside a chamber of a vacuum apparatus, an exhaust system is a cryo-cooling system. Further, there is a plurality of chambers separately used for film deposition and for taking in and out the substrate. This prevents the film deposition chambers from being exposed to atmosphere, and the chance of contamination of the substrate surface is reduced.
It is described that the aluminum film produced by the related art techniques and the aluminum film produced by the new technology are evaluated by: observing the film particles by diffraction characteristics graphs using an X-ray diffraction system and using an electron microscope, and by thermally treating the aluminum films and observe the surface state with a microscope. It is stated that a half-value width of the aluminum film formed by the new technology by a locking curve method is substantially equivalent to a half-value width of a bulk aluminum single crystal, and that the aluminum film is a high quality single-crystal film.
It is stated that, when about 20 milliwatts of power was supplied to both the SAW resonator formed by the related art technique and the SAW resonator formed by the new technology so as to observe frequency changes with time, the frequency change after 1,000 hours by the resonator of the new technology was minus several ppm, while the frequency change by the resonator of the related art was extremely large, from minus several tens to −100 ppm or more.
Patent Application Publication No. WO 00/24123 describes an aluminum electrode formed on an ST cut quartz substrate of a rotary Y plate obtained by rotating the Y plate by 10° to 60° around an axis X. Due to developments of measurement analysis technology, it is understood that the analysis on the aluminum film that used to be understood as single-crystal may have slightly changed. According to the disclosure of the publication, an aluminum layer is polycrystalline, and a crystal grain boundary is a twin crystal grain boundary (which is a crystal grain boundary in that adjacent crystal grains have a relation of twin crystals). It is stated that, accordingly, the frequency of the SAW resonator does not readily change even after a long-term use. This is because grain boundary energy is generated if the aluminum layer is polycrystalline. If the aluminum layer is made of poly crystal, as is the aluminum film of the related art, and if the orientation of each crystal constituting this poly crystal is random, the grain boundary energy increases. When the grain boundary energy increases, and when vibration is applied to the electrode film, the crystal moves gradually. Therefore, it is stated that, if such an electrode film receives vibration for a long period of time, it is conceivable that the film deteriorates with time and that the frequency fluctuates.
In contrast, if the grain boundary of the crystal constituting the aluminum film is the twin crystal grain boundary as in the present invention, that is, if the adjacent crystal grains have a relation of twin crystals, the grain boundary energy decreases, and particles constituting the aluminum film do not readily move even if vibration is applied to the electrode film for a long period of time. Thus, it is described as conceivable that the electrode film does not readily change with time even after a long-term use.
It is stated that a section of the SAW resonator was observed using a transmission electron microscope (TEM), and it was confirmed that the interface of the grain boundary of the aluminum film was the twin crystal grain boundary. Also, it is stated that the twin crystal was confirmed by a photograph taken using electron beam diffraction.
Patent Document 1: Japanese Examined Patent Application Publication No. 62-016050
Patent Document 2: Japanese Examined Patent Application Publication No. 1-034411
Patent Document 3: Japanese Unexamined Patent Application Publication No. 5-199062
Patent Document 4: WO00/24123
Non-Patent Document 1: Meirion Lewis, “Surface Skimming Bulk Wave, SSBW”, IEEE Ultrasonics Symp. Proc., pp. 744-752 (1977)